What are the default Frame / Axes styles and how can I find them?

Hunting through the stylesheets is effective, but does not necessarily give you the current value being used if it has been modified. Instead, use

CurrentValue[{StyleDefinitions , "GraphicsAxes"}]
(* {Arrowheads -> {}, LineColor -> GrayLevel[0.4], Thickness -> Absolute[0.2]} *)

CurrentValue[{StyleDefinitions , "GraphicsFrame"}]
(* {LineColor -> GrayLevel[0.4], Thickness -> Absolute[0.5]} *)

The styles "GraphicsAxes" and "GraphicsFrame" are defined in the stylesheet "Core.nb".

• To access to this file use the menu : format/edit stylesheet
• Then in the notebook that appears click on Default.nb
• In the new notebook that appears click on Core.nb
• Then the notebook core.nb appears.
• Go to section :"style for Mathematica System-specific Elements" and open it
• Go to the cell "default Box style" and open it
• Go to "Local definition for style Graphics"
• Select the cell "GraphicsAxes"
• Do Control-Shift-E (on Windows, otherwise you can use the menu : Cell/ShowExpression)

• Then you see :

  Cell[StyleData["GraphicsAxes"], Arrowheads->{}, LineColor->GrayLevel[0.4], Thickness->Absolute[0.2]]

This is the internal structure of the cell. One can see the settings for the style "GraphicsAxes"